With the recent rise of energy prices, further technical innovations are being sought in aircraft. That is, in the new model aircraft and the new model concepts recently announced by the Boeing Company (U.S.) and Airbus (France), the utilization of aluminum alloys such as extra super duralumin (designated as “A7075” according to the Japanese Industrial Standards (JIS)) and extra duralumin (similarly designated as “A2024”) for reducing the weight of the fuselage has been abruptly decreased and the utilization of carbon fiber reinforced plastic (abbreviated below as “CFRP”) is being increased by a corresponding amount.
A7075 (extra super duralumin), which is one type of aluminum alloy, has a specific gravity of about 2.7, whereas CFRP has a specific gravity of 1.6 to 1.7, making it incomparably lighter. Up until now, the utilization of CFRP as a material for aircraft has been increased in military applications such as fighter planes and combat helicopters, while use as a civilian aircraft material has not grown so much as anticipated due to the need to develop know-how on manufacturing large components and because of obstacles from the standpoint of cost. The use of CFRP has been avoided in civilian applications, in spite of their light weight, high strength and high corrosion resistance. Lately, however, in addition to research and development by the above two leading civilian aircraft manufacturers, the rise of the price of crude oil and its duration have made it imperative to examine ways of reducing the airframe weight.
The automotive industry as well, in order to cope with the rise of crude oil prices and environmental issues, is promoting the development of hybrid vehicles, electric cars and, in Europe, high-performance diesel engine vehicles. Moreover, it is expected that fuel cell vehicles, which have the highest energy conversion efficiency, will someday see widespread use. Although significant advances have been made in the development of fuel cells, the largest and most difficult problem that remains for use in automobiles is how to safely and smoothly handle hydrogen fuel, for which laborious work has been made. While safety is most important in civilian use, the obstacles are too large to make it a routine matter for general users to deal with cars loaded with hydrogen gas under several hundred atmospheres of pressure. Hence, the current liquid fuel system is not likely to change for the time being. In this sense, reducing the weight of vehicle construction is important for achieving real improvements in the power system. In fact, aluminum alloy materials are being applied increasingly in some types of vehicles and it is predicted that use of CFRP will be required someday.
In aircraft manufacture, the increased utilization of CFRP materials entails an increase in material expenses due to the high-cost CFRP materials, in addition to which it presents technical problems. One such problems is that, even in the latest Airbus and Boeing aircraft for which mass production has already been decided, it is estimated that the utilization of aluminum alloys will top 50% in terms of weight, with both CFRP materials and A7075 materials being used. CFRP materials will be used in areas close to the wing tips and A7075 materials will be used, as in the past, in central structure areas of the fuselage. Joining of the two materials will generally be done with rivets or with bolts and nuts, while special products will be required for this purpose.
This is because of the large differences between CFRP materials and metals in their basic properties. Metal materials have a large elongation (elongation at break), that of A7075 being 10 to 16%, whereas CFRP has an elongation of only several percent. When a strong tensile force is applied to these materials, the metal material undergoes elastic stretch (in accordance with Young's modulus, elongation and contraction that is proportional to the force) up to a certain strength; when a force that exceeds this limit is applied, elongation in excess of Young's modulus takes place, where break occurs at 110 to 116% of the original length (100%) for A7075. On the other hand, for CFRP, when pulled in a direction parallel to the fibers, elongation of the carbon fibers themselves is only 1 to 2%. When a large tensile load is applied that exceeds the range of elongation and contraction in accordance with Young's modulus, the carbon fibers break and the CFRP tears. In other words, CFRP has a small range in the ability of the material to absorb forces by undergoing elongation itself.
The same is true not only for tensile forces but also for crushing forces (compressive forces). That is, when tightened with bolts and nuts, even if the compressive forces exceed a certain limit, a metal will be deformed itself, enabling break to be avoided. However, when a compressive load is applied to CFRP, the force is first supported by the cured epoxy resin; with the application of an excessive pressing force, the epoxy resin is forced to spread peripherally and be deformed but spreading is limited by the carbon fibers, as a result of which the resin does not move and thus cannot be deformed, leading ultimately to failure.
In short, when a throughhole is formed in CFRP, a bolt is inserted into the hole and a nut threaded on the bolt is used to tighten the CFRP with an excessive torque, the CFRP undergoes compressive break. Because the large differences between the physical properties of both materials are due to inherent differences between a metal part, which is composed of metallic bonds between atoms, and epoxy resins and carbon fibers, which are composed of covalent bonds between carbon atoms and the like, there are basically no means for improving the properties themselves. Therefore, when both are tightened and secured with bolts and nuts, the only solution is to avoid applying excess pressure so as to prevent break on the CFRP side. This requires the development and use of special bolts and nuts. It has even been reported that the success by a certain corporation in developing such a bolt structure helped to spur the competition today in development of civilian aircrafts.
In future aircrafts, regardless of the degree to which the utilization of CFRP increases, the utilization of light metal materials will never go to zero, which means that techniques for easily joining CFRP materials with aluminum alloy materials will continue to be very important basic technology. There is another problem as to releasing of the CFRP prepreg from the metallic mold after its heat-curing. The prepreg undergoes heat-curing in a pressurized state under compressive forces applied by the metallic mold. In this operation, because the epoxy resin functions as an adhesive with respect to the mold as well, it is necessary to apply a release agent between the mold and the prepreg. As a result, infiltration of the release agent (generally a silicone oil-type release agent) into the CFRP product is unavoidable, which makes it impossible to achieve the highest physical properties inherent to the epoxy resin. Even minor decreases in quality are issues that must be resolved for use in structures for high-speed moving machinery such as aircrafts and automobiles. The present invention was conceived in order to provide a solution to such problems.
The inventors have made inventions as to techniques for securely joining a plastic part formed by injection molding with a metal part that has been inserted beforehand into the metallic mold for injection, specifically, aluminum alloy parts, magnesium alloy parts, copper alloy parts, titanium alloy parts, stainless steel parts, etc. (which techniques are referred to below as “injection joining” techniques) (see Patent Document 1: WO 03/064150 A1, Patent Document 2: WO 2004/041532 A1, Patent Document 4: Japanese Patent Application No. 2006-329410, Patent Document 5: Japanese Patent Application No. 2006-281961, Patent Document 6: Japanese Patent Application No. 2006-345273) and Patent Document 6: Japanese Patent Application No. 2006-354636).
A major factor was the discovery of surface treatment methods carried out beforehand on the metal to be inserted. The inventors anticipated that the surface shape of the metal obtained by such surface treatment would have a desirable effect not only on injection joining but also on joining (adhesion) with ordinary adhesives.
In the above-described inventions as to “injection joining”, the surface state desired in the metal alloy to be used can be summarized in terms of the following conditions (1) to (3). The first of these conditions (1) is that the rough surface obtained by chemical etching have irregularities (depressions and protrusions) with a period of 1 to 10 μm, with the height difference between the depressions and protrusions being about one-half of the period, i.e., 0.5 to 5 μm. The reason for this is that, when the molten resin flows into the metallic mold having a temperature well over a hundred and several decade degrees Centigrade lower than the melting point of the resin under a high pressure of several hundred to a thousand atmospheres, the diameter of the depressions into which the resin can somehow manage to penetrate as it crystallizes and hardens is 1 to 10 μm.
However, it is actually difficult that cover 100% of an aluminum alloy surface in this way with such a rough surface, given the variability of chemical reactions. It is practically regarded that such a surface roughness satisfies the above-mentioned roughness conditions, for which a curve indicating depressions and protrusions with an irregular period ranging from 0.2 to 20 μm and having a maximum height difference in a range of 0.2 to 10 μm as measured with a surface roughness tester can be traced, or for which a mean peak spacing (RSm) is 0.8 to 10 μm and a maximum height (Rz) is 0.2 to 10 μm analyzed with a scanning probe microscope, where such amounts as RSm and Rz are defined in the JIS standard (JIS B 0601:2001). The inventors, having recognized and concluded that the period of the depressions and protrusions on an ideal rough surface is, as noted above, 1 to 10 μm, refer in the present invention to a rough surface with a surface roughness defined in this way as a “surface with micron-order roughness,” which is a readily understood technical term.
The second condition (2) is that, when the rough surface is viewed with a magnification of an electron microscopic level, it has a finely irregular surface of depressions and protrusions with a period of 10 to 500 nm, most preferably a finely irregular surface with a period of 40 to 50 nm. The third condition (3) is that the surface is covered with a thin layer of metal oxide which is either thicker or stronger than an ordinary natural oxide layer for that particular metal alloy. Each of these three conditions required on the metal alloy side were attained, as noted above, for all of the following: magnesium alloys, titanium alloys, copper alloys, stainless steel alloys, aluminum alloys, etc., as a result of which it was possible to achieve a high shear breaking strength between the metal and the cured resin of at least 20 to 30 MPa with injection joining. This clearly proved the correctness of the hypothesis that the above three conditions are essential for injection joining. At this point, the inventors expected that this hypothesis should obviously have desirable effects also in joining (adhesion) with adhesives.
Hence, the inventors proposed the following hypothesis concerning adhesive joining. First, the metal alloy piece with a surface which satisfies the three above mentioned conditions is prepared in the same way as that used in the above described injection joining experiment and a liquid, one-pack epoxy adhesive is applied to the metal piece. Next, the metal piece is subjected to such steps as being placed in a vacuum state and then returned to standard pressure, causing the adhesive to infiltrate and smoothly coat the finely textured face of the metal alloy surface. The adhesive is then cured through heating. It was thought that, in this way, the liquid epoxy adhesive would be able to enter into the depressions of the micron-order roughness of condition (1) for the metal alloy surface at an inflow pressure of only about one atmosphere. If such penetration is possible, the epoxy adhesive will be cured within these valleys with subsequent heating.
In this case, the inner walls of these depressions have the nanometer-level finely irregular surface of condition (2). Complete penetration of the epoxy adhesive to the depths of the fine depressions formed in the finely irregular surface of (2) is presumed to be difficult. However, some of the epoxy adhesive does extend inside the openings of the fine depressions and is cured. In such a case, the epoxy adhesive that has been cured inside a large depression achieves the state of being secured (engaged) at the interior of the depression by innumerable spikes, which should make it difficult to be peeled away from the metal substrate by an external force. When the cured epoxy resin is forcibly peeled away, because the metal oxide layer of above-described condition (3) for the surface in contact with the spikes has a sufficient thickness to exhibit the hardness of this ceramic material, deformation on the metal alloy side is limited, making it impossible to pull out the epoxy resin within the large depression. Ultimately, the epoxy resin itself breaks near the opening of the large depression. In such cases, the force required for break far exceeds the adhesive strength data for hitherto known adhesives.
The correctness of this hypothesis has, in fact, already been proven, first for aluminum alloys, then for magnesium alloys, copper alloys, titanium alloys and stainless steel alloys. Subsequent to the present invention, the inventors are occupied in verification tests and plan to propose a group of inventions concerning various metal alloys. Based on these tests, the inventors believe that the above hypothesis concerning adhesive joining is correct, while it will accept the approval or criticism from many scientists and chemists. In the present invention, the inventors refer to this hypothesis as “NAT (Nano Adhesion Technology)”. With NAT, as explained above in regards to adhesion with an adhesive, joining can be understood as an entirely physical effect, that is, as an anchoring effect. Without such an understanding, it is impossible to explain the immense joining strengths of 500 to 700 kgf/cm2 (50 to 70 N/mm2=50 to 70 MPa), measured as the shear strength at break and the tensile strength at break, which are exhibited with the use of an epoxy-based adhesive, not only for aluminum alloys but also for other metal alloys as well.
In addition, it has been mentioned above as condition (1) that large depressions and protrusions, that is, depressions and protrusions with a period of 1 to 10 μm, are desirable, while this NAT applies not only to the aluminum alloys shown in the present invention but it has been demonstrated also, for example, for copper alloys, titanium alloys, stainless steel alloys and common steel materials. In cases where depressions and protrusions larger than described above are present and, conversely, in cases where depressions and protrusions that are too much smaller than described above are present, the joining strength due to joining with an adhesive became lower. For depressions that are too large, the most likely reason is such that the density of depressions that forms per unit surface area becomes low, thus reducing the anchoring effect. For depressions that are too small, penetration of the epoxy adhesive into the interior is probably insufficient.
By utilizing the above-described joining strength according to NAT, it is possible to achieve the many needs mentioned in the preceding section. First, in the case of adhesive joining between metal alloys for which surface treatment according to NAT hypothesis was made, a very strong adhesive force can be obtained between metal alloys by using an epoxy adhesive. This is the case both in joining between aluminum alloys and in joining between an aluminum alloy and a titanium alloy. The reason is that the joining strength itself does not arise between the metals but it rather arises between the respective metals and the epoxy resin. Also, a fiber-reinforced plastic (FRP) material in which the epoxy resin is used as the matrix is, not surprisingly, the most trouble-free mating material for adhesive joining with a metal alloy piece. It can be appreciated that, by pressing together a FRP prepreg and an aluminum alloy part that has been coated with an epoxy adhesive, raising the temperature and simultaneously curing the epoxy resins on both sides, joining (anchoring) will be easier than between metals.
One conceivable form of this adhesion is a structure in which the FRP material is sandwiched between thin sheets of aluminum alloy, that is, a laminated structure. Although there is an increase in weight, degradation of the epoxy resin by a release agent can be prevented, because a mold release agent for the prepreg is not necessary. Also, in cases where a sandwich structure is formed in which part of the prepreg rather than the entire surface is placed between thick sheets of aluminum alloy to form a sandwich structure, throughholes are formed in the structure, bolts are passed through the holes and joining to another part is carried out, break of the CFRP portion can be avoided even when the bolts are over-tightened.
Additionally, in a plate-like or tubular structural member formed integrally of an aluminum alloy member at the ends and a CFRP member as the main material at the center, connection with bolts and nuts, fitting or some other known metal joining method may be employed by utilizing the ends thereof, facilitating assembly and disassembly, thus giving a member compatible with large-volume production. This should be helpful in reducing weight and increasing strength not only in aircrafts but also in vehicles such as automobiles, in mobile electronic and electrical equipments, in robot machineries or the like. Today, CFRP materials are very familiar materials. The ability to employ such materials in vehicle applications will enable major contributions to be made to the energy-saving and environmentally responsible society in the future.